I. Field
The present invention relates generally to wireless communication networks, and more specifically to provisioning spectrum resources between wireless networks.
II. Background
The background description includes information that may be useful in understanding the present inventive subject matter. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed inventive subject matter, or that any publication, specifically or implicitly referenced, is prior art.
Fading and interference are the two key challenges faced by designers of wireless communication systems. While fading limits the coverage and reliability of any point-to-point wireless connection, e.g., between a base station and a mobile terminal, interference in prior-art networks restricts the reusability of the spectral resource (time, frequency slots, multiple-access codes, etc.) in space, thus limiting the overall spectral efficiency.
Two recent research trends in wireless networks offer potential solutions: very dense spatial reuse and multi-user multiple input, multiple output (MU-MIMO). Dense deployment of infrastructure (e.g., more WiFi access points, more cell towers, micro-cells, pico-cells, and femto-cells, etc.) is intended to enable multiple short-range low-power links to co-exist on the same time-frequency channel resource. However, this approach presents its own limitations, as denser infrastructure results in more interference. In cellular systems, increased inter-cell interference establishes a practical upper bound for cell density.
MU-MIMO refers to serving multiple users on the same time-frequency resource, usually by eliminating multi-user interference via spatial precoding. In theory, an access point with a sufficient number of antennas can simultaneously serve an increasing number of clients while maintaining a constant per-client data rate. In practice, such systems suffer from diminishing returns. Specifically, the channel between the clients and the access point requires sufficiently rich scattering in order to realize spatial multiplexing gains, which means the access point's antennas need to be sufficiently separated. Due to the limited space at an access point or cell site, additional antennas cause more correlation between channels, resulting in less capacity benefit for each additional antenna. In such cases, the ratio of cost to spectral efficiency rises exponentially.
In 2001, Distributed MU-MIMO was introduced (S. J. Shattil, Pat. Appl. Ser. No. 60/286,850, filed Apr. 26, 2001), which unifies these two approaches, simultaneously obtaining both multi-user interference suppression via spatial precoding and dense coverage by reducing the average distance between transmitters and receivers. This is achieved by coordinating a large number of access points (e.g., base transceiver terminals) distributed over a certain coverage region via a wired, optical, or wireless backhaul network connected to a central processor in order to form a distributed antenna system. Thus, the multiple access points can function together as a single distributed access point, referred to as a “super array.” In subsequent patent filings, Applicant disclosed this idea, termed “cooperative” (also known as collaborative, distributed, or virtual) MIMO, with respect to all types of wireless terminals in a radio access network (RAN), as well as various network configurations, including cellular, ad-hoc peer-to-peer (e.g., device-to-device (D2D)), mesh, and hierarchical network topologies.
In 2002, Software-Defined Radio (SDR) in Distributed MU-MIMO and Cloud Radio Access Networks were introduced (S. J. Shattil, patent application Ser. No. 10/131,163 filed Apr. 24, 2002 (now U.S. Pat. No. 7,430,257) and S. J. Shattil, patent application Ser. No. 10/145,854, filed May 14, 2002). Solutions to synchronization and calibration in Distributed-MIMO are presented in the '257 patent, the '854 application, S. J. Shattil, Pat. Appl. Ser. No. 60/598,187, filed Aug. 2, 2004, and S. J. Shattil, patent application Ser. No. 11/187,107 (now U.S. Pat. No. 8,670,390). All the references mentioned in this disclosure are incorporated by reference in their entireties.
As explained above, a key problem faced by cellular network providers is that legacy infrastructure costs increase exponentially relative to capacity, which means that providers don't benefit from economies of scale. Also, legacy network costs increase exponentially with the data rate per user. Thus, cellular providers have a strong economic incentive to delay network upgrades and new services. In addition to lagging behind the civilized world (such as Mexico and Russia) in cellular network speed and affordability, America's speeds are declining while the rest of the world is improving.
While American cellular providers complain that they no longer have enough spectrum to meet the demand for data, the fact is that any technical advantage one provider might gain over its competitors could be negated by its obligations under cross-licensing agreements with the other providers. This stifles innovation and keeps prices high, leaving consumers stuck with Third-World-quality service. Furthermore, cellular providers are locked into long-term contracts with hardware manufacturers that strongly discourage, if not outright prohibit, third-party improvements to the network infrastructure.
Similarly, Internet service in America lags behind the civilized world because the average market has only one or two ISPs who set their prices at monopoly or duopoly pricing. The slow speeds and lack of competition are due, in part, to the high cost of providing optical fiber to the home (i.e., the last-mile), and deploying such networks takes years. Even though Cooperative-MIMO technologies mentioned above provide an economically compelling alternative to legacy cellular and ISP infrastructure, the main barrier to entry for entrepreneurs is the high cost of licensed spectrum.
Cognitive radio has been proposed to address the inefficient use of licensed spectrum. Portions of the spectrum that are not in active use by licensed users are called white spaces, which cognitive radio seeks to use. Transmission and reception parameters require an efficient medium access control layer to utilize the dynamic white spaces in order to avoid interfering with other devices. However, conventional medium access control employs contention-access mechanisms that lead to poor spectrum efficiency.